The present invention generally relates to determining the make-up of subscriber loops in the public switched network and more specifically to methods and systems that determine the make-up of subscriber loops via single ended measurements.
The mainstay of the telephone company local network is the local subscriber loop. The great majority of residential customers, and many business customers, are served by metallic twisted pair cables connected from a local switch in the central office to the subscriber""s telephones. When customers request service, request a change in service, or drop service, these facilities must be appropriately connected or arranged in the field, referred to as the xe2x80x9coutside plant,xe2x80x9d and telephone companies have specially trained craft dedicated full time to this task. Obviously a company needs to have an understanding of its subscriber loops including where they are connected and the location of the flexibility points such as junction boxes, etc. These records historically were kept on paper, called xe2x80x9cplats,xe2x80x9d and more recently are manually entered into a computer database. However, even when entered into a database there are still problems associated with keeping the records accurate and up-to-date.
Having accurate records of the loop plant is critically important to many aspects of a telephone company""s business. In addition to the need for accurate records to provide traditional voice services, there will be a need for even more accurate and more detailed records in order to deploy a whole new class of xe2x80x9cxDSLxe2x80x9d based services, including those based on integrated services digital network (ISDN), high-rate digital subscriber line (HDSL), asymmetrical digital subscriber lines (ADSL) and very high rate digital subscriber lines (VDSL) technology. These technologies are engineered to operate over a class of subscriber loops, such as nonloaded loops (18 kft), or Carrier Serving Area (CSA) loops (9 to 12 kft). In fact, the need to be able to xe2x80x9cqualifyxe2x80x9d a loop for provision of one of these technologies is becoming critical, as the technologies emerge and deployment begins. The ability to easily and accurately qualify loops will allow telephone companies to offer a whole range of new services; problems and high expenses associated with qualifying loops can potentially inhibit deployment and/or lower or forego associated new revenues. The unscreened multipair cables in the existing subscriber loop network constitute the main access connection of telephone users to the telephone network. Recently, the demand for new services such as data, image and video has increased tremendously, and telephone companies have planned to deliver broadband ISDN services via fiber optic local loops. However, the deployment of fiber optic cables in the access plant will require at least twenty years, so that, in the meantime, it is extremely important to fully exploit the existing copper cable plant.
Although there are many different digital subscriber line services, for example, ISDN basic access, HDSL, ADSL, VDSL, and Synchronous DSL (SDSL), these services are not always available to every customer since copper lines seem to present more problems than expected. In fact, the cable length and the presence of load coils and bridged taps may deeply affect the performance of DSL services. Unfortunately, loop records are unreliable and often don""t match the actual loop configuration, so that the existing databases cannot be fully exploited.
Loop prequalification is an important issue not only because it can help an economic deployment of DSL services, but also because it can help telephone companies in updating and correcting their loop-plant records. From this point of view, the feasibility of accurate loop make-up identification would have a much higher economic value than simple DSL qualification.
One way to obtain accurate loop records is to manually examine the existing records and update them if they are missing or inaccurate. This technique is expensive and time consuming. Furthermore, new technologies such as xDSL require additional information that was previously not kept for voice services, so there is the potential that new information needs to be added to all existing loop records. Test set manufacturers offer measurement devices that can greatly facilitate this process, but typically they require a remote craft dispatch.
Another way to obtain accurate loop records is by performing a loop pre-qualification test. There are essentially two ways of carrying out a loop pre-qualification test: double ended or single ended measurements. Double-ended measurements allow us to easily estimate the impulse response of a loop by using properly designed training sequences. Double-ended testing, however, requires equipment at both ends of the loop. Specifically, in addition to equipment at the Central Office (CO) or near end of the loop, double ended testing involves either the presence of a test device at the far end of the loop (Smart Jack or MTU) or dispatching a technician to the subscriber""s location (SL) to install a modem that communicates with the reference modem in the CO. An exemplary double ended system and method that extrapolate voice band information to determine DSL service capability for a subscriber loop are described in Lechleider et al. U.S. Pat. No. 6,091,713, issued Jul. 18, 2000, entitled xe2x80x9cMethod and System for Estimating the Ability of a Subscriber Loop to Support Broadband Servicesxe2x80x9d (which is assigned to the assignee of the present invention).
In contrast, single ended tests are less expensive and time consuming than double-ended tests. Furthermore, single-ended testing requires test-equipment only at the CO. In fact, no technician dispatching is required and the CO can perform all the tests in a batch mode, exploiting the metallic access with full-splitting capability on the customer""s line. An example of such a single ended test system is the xe2x80x9cMLTxe2x80x9d (Mechanized Loop Testing) product that is included as part of the widely deployed automated loop testing system originally developed by the Bell System. The MLT system utilizes a metallic test bus and full-splitting metallic access relays on line card electronics. By this means, a given subscriber loop can be taken out of service and routed, metallically, to a centralized test head, where single-ended measurements can be made on the customer""s loop. The test head runs through a battery of tests aimed at maintaining and diagnosing the customer""s narrowband (4kHz) voice service, e.g., looking for valid termination signatures via application of DC and AC voltages. This system is highly mechanized, highly efficient, and almost universally deployed. In addition, the MLT system is linked to a Line Monitoring Operating System (LMOS) thereby providing a means to access and update loop records which are useful in responding to customer service requests or complaints. However, because this system exclusively focuses on narrowband voice services, the system misses important loop make-up features that will be deleterious to supporting broadband services via DSL technologies.
Another well known single-ended measurement technique relies on the observation of echoes that are produced by medium discontinuities to fully characterize the link. Specifically, these single ended measurements typically rely on time domain reflectometry (TDR). TDR measurements are analogous to radar measurements in terms of the physical principles at work. TDR test systems transmit pulses of energy down the metallic cable being investigated and, once these pulses encounter a discontinuity on the cable, a portion of the transmitted energy is reflected or echoed back to a receiver on the test system. The elapsed time of arrival of the echo pulse determines its location, while the shape and polarity of the echo pulse(s) provide a signature identifying the type of discontinuity that caused the reflection or echo. Basically, if the reflecting discontinuity causes an increase in impedance, the echo pulse""s polarity is positive; if the reflecting discontinuity causes a decrease in impedance, the echo pulse""s polarity is negative. A bridged tap, for example, produces a negative echo at the location of the tap and a positive echo at the end of the bridged tap. Accordingly, a trained craftsperson is able to determine the type of fault based on the shape, polarity, and sequence of pulses.
Nevertheless, TDR methods (or, in general, single ended measurements that rely on echo pulse signatures) are inaccurate and provide ambiguous results that even the most skillful craftsperson cannot interpret. Because the arrival of echoes is dependent on the location of the discontinuities (or faults) one echo can be masked by another echo if the echoes overlap. For example, FIG. 1A illustrates an exemplary loop having three discontinuities, two of which are bridged taps 500 feet apart. In accordance with TDR methods, a pulse 10 is sent from CO 13 to subscriber location 15 on the subscriber loop segments 20, 22, 24, and 26. As the pulse traverses the loop, it encounters a gauge change 30, a first bridged tap 32, and a second bridged tap 34 before arriving at SL 15. FIGB. 1B depicts the echo pulses caused by the bridged taps 32 and 34 (the echoes generated at the gauge change 30 and SL 15 were filtered out from FIG. 1B). As FIG. 1B shows it is not possible to detect the two-bridged taps via prior art TDR methods. In fact, looking at FIG. 1B, it appears as if only one bridged tap 32 is present since there is only one negative 70-positive 80 transition. However, since the positive echo 80 is not weaker than the negative one 70 (as it usually is for bridged taps), it may be induced that either the bridged tap is very short or several bridge taps are present. However, since the width of the positive echo 80 looks very large, it is very unlikely that it was a short bridged tap since a short bridged tap would introduce a small amount of distortion and, consequently, a narrower pulse. Therefore, the case of several bridged taps may be the most probable, although we cannot say how many there are. As such, TDR methods can produce ambiguous results.
In addition, prior art TDR methods do not take into account and, more specifically, are unable to isolate the effects of spurious pulses. That is, with reference to FIG. 1A, as pulse 10 arrives at gauge change 30, a portion of pulse 10 is reflected to generate a first real echo, and the remaining portion (or refracted portion) travels toward bridged tap 32. At bridged tap 32, reflection and refraction again occur in the process of producing a second real echo. This second echo pulse (traveling upstream to CO 13) is then reflected at gauge change 30 back to bridged tap 32 where a spurious echo pulse is reflected to the CO 13. Although spurious echoes will be more attenuated than real echoes, they are added and overlapped to the real echoes causing the real echo signals to be distorted. Accordingly, spurious echoes enhance the ambiguity inherent in T DR measurements because the shape of the echo is used to interpret the type of fault that causes the echo. In other words, a craft person interpreting a TDR measurement analyzes a distorted trace that does not distinguish spurious echo distortion. More importantly, the effects of spurious echoes on the pulse shape cannot always be interpreted via human visual inspection.
Of utility then is a method and system for unambiguously and completely determining a subscriber loop make-up including detecting the presence and location of load coils, gauge changes, and bridged taps and the length of the loop including the length of each bridged tap.
Our invention is a method and system for unambiguously and completely determining a subscriber loop make-up.
In accordance with an aspect of our invention the narrowband test head and associated measurements that are included as part of prior mechanized loop test system may be augmented and/or replaced with a broadband test head and associated measurements. By using a broadband test head and signal processing algorithms our invention advantageously allows automated measurements of any subscriber loop that completely determine the loop make-up.
In accordance with an object of our invention, existing loop records can be checked. Specifically, because our invention can be automated, loop make-ups can be methodically obtained and then compared to existing database records. More likely, because we expect the information to be more accurate and detailed than the existing records, the new data will simply replace existing data and populate the database. Thus, through an entirely mechanized process, a telephone network or service provider can completely update its records. Having an up-to-date database will help the provider in its day-to-day business operation, and may also position it to cost-effectively meet requirements imposed as a result of xe2x80x9cloop unbundlingxe2x80x9d rulings resulting from the Telecom Act of 1996.
In accordance with an additional object of our invention, loop make-ups can be advantageously used by another mechanized system to calculate and qualify the ability of a given loop to support advanced DSL services. A computer system can take the loop make-ups, models of the various DSL systems, and standards for spectral compatibility and determine very precisely the service level of a given subscriber. Such determination is likely to be achievable in real-time, for example in response to a service agent""s query while on the phone with a customer.
In accordance with another object of our invention, additional information will be determined regarding the noise environment a particular loop is exposed to. This noise may be analyzed and identified, further facilitating the service provisioning process, identifying noise sources exceeding spectral compatibility requirements, and generally facilitating maintenance and administration.
Our broadband test head method includes the steps of sending at least one pulse on the loop, receiving the echoes generated when the pulse encounters a discontinuity, and processing the received echoes to determine the type and location of each discontinuity. Our processing method starts at the central office having access to the loop and moves along the loop to the subscriber location. As we move out along the loop we compute the transfer function for each preceding loop section (excluding bridged taps), synthesize a filter based on the transfer function, and convolve the synthesized filter with the echo data.
Our invention can be implemented in either a distributed or non-distributed embodiment or architecture. In the distributed architecture the functions of data acquisition and processing are done at different locations, whereas in the non-distributed architecture each of these functions are done at the same location, preferably by the same equipment. In either embodiment our method can be used to completely determine the loop make-up.